Cochlear outer hair cells are a class of sensory cells in the ear that convert sound-induced mechanical vibration into electrical signal. These cells also act as a fast motor capable of responding at auditory frequencies. The sensitivity and the sharp frequency discrimination of the mammalian ear is achieved by pumping energy into mechanical resonance of the basilar membrane. We have previously established that the hair cell motor uses electrical energy based on piezoelectricity, having electric charge that is transferable across the membrane and membrane area changes associated with it. We determined the membrane area changes of prestin, a membrane protein that is an essential component of the motor. The area changes were less than half of those of outer hair cell motor. The result implies that prestin is indeed a motor protein and that its action is likely amplified by interacting with other membrane proteins. Force produced by outer hair cells depends on the voltage oscillation (receptor potential) in the cells due to transducer current in hair bundles. Studies on electric properties of these cells indicate that the receptor potential is highly attenuated because the most part of the transducer current is used for charging up and down the membrane that acts as a capacitor. As the result, receptor potential has been regarded as too small to affect vibration in the cochlea. We found that the receptor potential at the resonance frequency should not be heavily attenuated up to 10 kHz because of piezoelectric resonance. The frequency limit arises from the condition that force produced by outer hair cells needs to cancel out viscous drag. The effective frequency range can be extended by fast potassium channels.